1. Field of the Invention
The present invention relates to a method of manufacturing metal bellows built in, for example, a steering system for an automobile.
2. Related Background Art
Bellows are components which have been used for various kinds of machinery. A bellows consists of alternately formed large- and small-diameter portions, and thus its cross section in its axial direction has corrugated shape. A bellows made of metal has relatively high flexural rigidity. When a strong impact in the axial direction is given, it can collapse to absorb the energy of the impact. As the metal bellows has such characteristics, collapsible steering systems for automobiles which use metal bellows as steering columns are being developed.
A metal bellows to be built in a collapsible steering system requires to have greater rigidity than those to be used for piping, and so on, where only flexibility is required. Such metal bellows having high rigidity have been manufactured according to the methods illustrated in FIGS. 18 and 19.
The method illustrated in FIG. 18 is described in "Working Method of Piping" (M. Nakamura, Nikkan Kogyo Shinbun Pub. p96). According to this method, an unprocessed metal tube 1 is charged with high-pressure liquid, and certain portions of the unprocessed tube 1 are bulged radially outward by the pressurized liquid to form a metal bellows 2. Note that, in FIG. 18, the shape of half of the unprocessed tube 1 is shown in the lower half of the figure and that of the finished metal bellows 2 in the upper half of the figure.
In the method of FIG. 18, the unprocessed tube 1 is set inside a work cylinder 3 and charged with high-pressure liquid such as water, oil, and the like, whereby the inner peripheral surface of the tube 1 is pressed outwards to expand. At the same time, work pistons 4a and 4b engaged in respective end portions of the work cylinder 3 are strongly pressed by the pressurized liquid toward each other. Bracing rings 5 which are arranged inside the work cylinder 3 with a certain interval therebetween in the axial direction can shift inside the work cylinder 3 in the axial direction (horizontally, in FIG. 18).
When pressurized by the liquid, certain portions of the unprocessed tube 1 are expanded radially outward as shown in the upper half of FIG. 18. At the same time, the overall length of the tube 1 is reduced. As a result, the metal bellows 2, the upper half of which is shown in FIG. 18, is formed.
FIG. 19 shows another method of manufacturing a metal bellows having high rigidity, which is disclosed in Japanese Laid-Open Patent Application No. 63-157724. According to this method, a portion of the unprocessed tube 1 with respect to the axial direction is subjected to Joule heating by a high frequency induction coil 6 while the tube 1 is pressed in the axial direction. Thus, the heated portion subjected to Joule heating is plastically deformed so as to be expanded radially outwards. Then, the high frequency induction coil 6 is shifted in the axial direction by a predetermined length, and the above-mentioned process is repeated. By repeating said process several times, large-diameter portions and small-diameter portions are alternately formed to obtain a metal bellows with a corrugated cross section in the axial direction.
The above-mentioned conventional methods are subject the following problems 1 and 2.
1: In the first method illustrated in FIG. 18, not only is the equipment expensive, but also it is difficult to manufacture a metal bellows which has sufficient rigidity to be used in the collapsible steering system of energy absorption type.
More specifically, as the unprocessed metal tube 1 is plastically deformed by the pressurized liquid, considerably high pressure of the liquid has to be generated. So, the hydraulic pressure generator becomes expensive. Besides, since the unprocessed tube 1 used for forming the metal bellows having high rigidity should be thick enough, it is difficult for the pressurized liquid to give sufficiently large force to plastically deform the tube 1. In addition, as liquid is used in the process, the operation is difficult to simplify and automate, thereby raising the manufacturing cost of the metal bellows.
2: In the method shown in FIG. 19, on expensive high frequency induction coil 6 is needed, which raises the cost of equipment. Also as the plurality of large diameter portions are expanded one by one, the work efficiency is not good. Therefore, also in this method, the manufacturing cost is high. Moreover, since the unprocessed tube 1 to be plastically deformed is heated, it is difficult to realize high precision in size and shape of the plastically deformed portions, which may deteriorate the overall precision of the finished metal bellows.